A group III nitride, for example, a gallium nitride (GaN)-based semiconductor is expected as a material for a next-generation power semiconductor device. The GaN-based semiconductor has a band gap wider than that of a silicon (Si) semiconductor. For this reason, in the GaN-based semiconductor device, the power semiconductor device can be made smaller in size and higher in breakdown voltage compared to the Si semiconductor device. Accordingly, a parasitic capacitance can be reduced, and thus a high-speed driven power semiconductor device can be realized.
In a GaN-based transistor, generally, a high electron mobility transistor (HEMT) structure which uses a two-dimensional electron gas (2DEG) as a carrier is applied. A normal HEMT is a normally-on transistor which is conducted even when a voltage is not applied to a gate.
In a power supply circuit or the like which handles a large amount of electric power of several hundreds V to one thousand V, a normally-off operation is required with emphasis on safety. In this regard, a circuit configuration is proposed in which the normally-on GaN-based transistor and the normally-off Si transistor are cascode-connected to realize the normally-off operation.
In the case of a circuit configuration in which a main circuit current flowing between a drain and a source and a drive current flowing between a gate and a source share a source inductance, the drive current is modulated due to the influence of an electromotive force which is generated in the source inductance according to the time change of the main circuit current. Accordingly, there occurs a problem of a delay that a rising speed or a falling speed of the power semiconductor device is slowed down or a ringing that a drain current or a source voltage is sharply changed with time. In this regard, a circuit configuration with Kelvin connection is proposed in which the main circuit current and the gate drive current do not share the source inductance.
However, even in the circuit configuration using Kelvin connection, the problem that the drive current is modulated due to the influence of the main circuit current cannot be resolved completely, and the problem that the rising speed or the falling speed of the normally-on transistor is slowed down still occurs. The decrease of the rising speed or the falling speed of the normally-on transistor may cause the increase of the switching loss. Thus, proper measures are required.